This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Intense, ultrashort laser pulses provide us with a way to access non-equilibrium states in solids by excitation of coherent phonons. Because the phonons in the material determine the mechanical and thermal properties, it is of great interest to develop tools to study the dynamics of the nonequilibrium phonon population induced by the laser. In addition, laser excitation can be used to access other nonequilibrium phases of matter through, for example, novel phase transitions involving lattice instabilities at high wavevector. Because of the long wavelength of visible radiation, only a small volume of the Brillouin zone is accessed in optical experiments. There is currently no experimental technique that can provide time-resolved information of the complete phonon branches of the solid. Time-resolved x-ray diffuse scattering (TRXDS) has the potential to be the ultimate tool to study these nonequilibrium processes throughout the Brillouin-zone of the crystal. We propose to use time-resolved diffuse X-ray scattering to study the lattice instability of Si and GaP induced by intense laser irradiation. The proposed experiment serves as a test of the feasibility of time-resolved x-ray diffuse scattering at APS and could potentially benefit a large part of the community with the advent of novel sources as the LCLS.